1. Field of the Invention
The present invention relates to a device and method for measuring glenohumeral (shoulder) joint translation in the anterior-posterior directions and the inferior direction.
2.Description of the Related Art
The shoulder is the most mobile joint in the body. This mobility allows a significant amount of motion that gives us the ability to throw and perform other overhead activities. Proper shoulder function requires the humeral head to remain relatively centered within the glenoid concavity during active motion. Damage to the soft tissue structures that maintain the joint in place typically leads to an excessive amount of motion, eventually progressing to either a dislocation, where the joint comes completely out, or a subluxation where the joint only partially comes out of place.
The term glenohumeral instability is used to define the symptomatic inability to maintain the humeral head centered in the glenoid fossa. In addition to a detailed history of the patient""s problem, a physical examination is the key to successful diagnosis of these problems. In examining a loose joint, the term glenohumeral laxity is used to describe the range of movement of the center of the humeral head with respect to the glenoid concavity while force is applied to the arm.
In cases of suspected shoulder instability, determining the degree of shoulder laxity in the involved shoulder and comparing it with that of the uninvolved shoulder may be helpful in making a diagnosis and determining the necessary treatment. While several authors have described techniques for evaluating and quantifying glenohumeral laxity (Gerber, Hawkins, Matsen), there is no data on clinical measurement of the absolute amount of translation which occurs on examination. (See Gerber C, Ganz R: Clinical assessment of instability of the shoulder. With special reference to anterior and posterior drawer tests. J Bone Joint Surgery, 66B: 551-556, 1984; Hawkins R J, Bokor D J: Clinical evaluation of shoulder problems, in Rockwood Cailf., Matsen F A III (eds): The Shoulder. Volume 1. Philadelphia, WB Saunders Co. 1990, pp 149-177; Matsen F A, Thomas S C, Rockwood Cailf.: Anterior glenohumeral stability, in Rockwood Cailf., Matsen F A III (eds): The Shoulder. Volume 1. Philadelphia, WB Saunders Co. 1990, pp 547-551.)
In the knee, several instruments are available to quantify translation of the joint in order to diagnose instability. These instruments are, for the most part, portable and easy to use by a variety of medical personnel. In sharp contrast to the knee, however, no comparable instrument has been developed to quantify joint laxity in the shoulder. Some studies are available on the absolute measurement of translation. However, these studies have employed instruments that either are to be used by personnel that are highly-skilled in their use, or the instruments are complex and not portable.
One study by Krarup, et al., evaluated the motion of the glenohumeral joint using an ultrasonic transducer. In their study, translation was studied in the anterior to posterior direction. They studied 20 healthy shoulders and 20 with instability. The average translation of the healthy shoulders was 1.9 mm, while that of the unstable shoulders was 4.9 mm. Furthermore, the difference from side to side in those with normal shoulders was 0.7 mm, while the difference in those with instability was 2.8 mm. The authors used a translation force of 90 Newtons to elicit the values quoted. While the variation by one examiner was not significant, they found that the variation between various examiners was significantly different. In summary, this technique certainly allowed for measurement of translation, however, it would need to be performed by a trained ultrasound technologist. (See Krarup A L, Court-Payen M, Skjoldbye B, Lausten G: Ultrasonic measurement of the anterior translation in the shoulder joint. Journal of Shoulder and Elbow Surgery, 8:136-141, 1999.)
Another study by Sidles, et al., has evaluated the amount of glenohumeral translation using a spatial tracking system. In normal shoulders, they found an average translation of between 8 and 11 mm, depending on the direction of testing. The system they employed was one that requires markers to be placed on the patient and the use of several video cameras to record the degree of translation. (See Sidles J A, Harryman D T, Harris S L, Matsen F A: In vivo quantification of glenohumeral stability. Orthopaedic Research Society, 37th Annual Meeting, Anaheim, Calif., March 4-7, 1991.)
Finally, Jorgensen and Bak have employed a modified knee laxity tester in the assessment of anterior-posterior (AP) translation of the glenohumeral joint. They found that measurements in this direction were highly reproducible. Additionally, the translation in normal patients was significantly less than in those with instability. In their study a force of 20 pounds was used to reproduce the given translations. This is equivalent to 85 Newtons. This device, however, measured only one plane of motion. (See Jorgensen U, Bak K: Shoulder instability. Assessment of anterior-posterior translation with a knee laxity tester. Acta Orthopaedica Scandinavica, 66: 398-400, 1995.)
U.S. Pat. No. 5,911,695, issued to Watkins, et al., discloses a device for testing the inferior glenohumeral ligament of the shoulder of a patient with the application of a measurable force. This device can measure the anterior to posterior translation of the humeral head relative to the clavicle while a measured force is applied. By virtue of its design, however, it allows for measuring only anterior to posterior motion and cannot be employed to measure any other direction of translation.
As will be disclosed below, the present invention measures the amount of translation that a glenohumeral joint undergoes as a specific force is applied to the arm. The directions of motion that can be quantified include the anterior-posterior and inferior directions. As used herein, the term xe2x80x9canterior-posteriorxe2x80x9d refers to directions from both the anterior to posterior and from the posterior to anterior. These directions are all important in the measurement of glenohumeral joint laxity.
It is therefore a principal object of the present invention to measure translation of the glenohumeral joint as a specific force is applied to the arm in the anterior-posterior and also the inferior directions.
It is another object to provide highly reproducible measurements of the translation of the glenohumeral joint resulting from consistent forces applied thereto.
Yet another object is to provide reproducible measurements of a glenohumeral joint translation as between varying examiners.
Still another object is to provide a measurement device that is portable.
These and other objects are achieved by the present invention which is a shoulder arthrometer for measuring glenohumeral joint translation upon application of a force to an arm of a patient. In its broad aspects it includes an arm attachment assembly, a force measurement gauge, a translation measurement gauge assembly, and a girdle assembly. The arm attachment assembly is capable of being secured to the arm of a patient. The force measurement gauge is operably engaged with the arm attachment assembly for measuring a force applied to the arm attachment assembly in anterior-posterior directions and the inferior direction. The translation measurement gauge assembly is operably engaged with the arm attachment assembly. It is capable of measuring anterior-posterior translations and inferior translations relative to the arm of a patient and includes a girdle connecting element. The girdle assembly is operably engageable with the girdle connecting element. The girdle assembly is capable of being secured to the torso of the patient to provide a fixed point of reference. During application of either an anterior-posterior force or an inferior, force to an arm of a patient, glenohumeral joint displacement is measured. The amount of force applied measurement gauge, while the amount of translation is registered via the translation measurement gauge assembly.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.